A waterbed includes an electrical heating element or pad which is arranged to heat the bed and in particular the water mattress thereof and to maintain the bed at a predetermined or "set" temperature. The heating element is controlled by a thermostat controller or control unit which comprises a temperature sensor arranged to sense the temperature of the bed. The controller effects switching on and off of the heating element to maintain the bed at this predetermined set temperature.
Experience shows that the optimum set temperature comfort-wise is not the same for all temperatures of the ambient environment external to the bed, in the bedroom. In wintertime, for example, when the temperature of the bedroom environment is reduced, users often wish to increase the "set" bed temperature slightly, to achieve maximum comfort, to overcome inter alia the increased heat loss from a top of the bed. In summertime, when the temperature of the bedroom environment is increased, users often wish to reduce the set temperature of the bed by a degree or two to create a comfortable cooling effect. It has been found that a typical median "set" temperature for a waterbed is between 25.degree. and 35.degree. Celsius. Userse typically like this set temperature to be increased or decreased by something of up to the order of two degrees for corresponding alterations over an environmental temperature range of between about 0.degree. to 40.degree. C.